replaceRouteBelow<T extends Object> method

Replaces a route on the navigator that most tightly encloses the given
context with a new route. The route to be replaced is the one below the
given anchorRoute.

The old route must not be current visible, as this method skips the
animations and therefore the removal would be jarring if it was visible.
To replace the top-most route, consider pushReplacement instead, which
does animate the new route, and delays removing the old route until the
new route has finished animating.

The removed route is removed without being completed, so this method does
not take a return value argument.